1. Field of the Invention
The present invention relates to a toner used for developing an electrostatic image formed in a process for forming an image, such as an electrophotographic process, an electrostatic recording process, and an electrostatic printing process.
2. Description of the Related Art
A full-color copier includes four image bearing members and an endless intermediate transfer member. The full-color copier forms a desired color image by: forming an electrostatic image on each of the image bearing members; developing the formed electrostatic images using a cyan toner, a magenta toner, a yellow toner, and a black toner, respectively; transferring toner images of the respective colors formed by the development on a transfer material such as an overhead transparency sheet (so-called OHP sheet) or a piece of ordinary paper so as to finally overlap the toner images; and fixing the toner images overlapped on the transfer material to the transfer material.
Therefore, the color toners are required to have a transparency enough to prevent the color of an upper layer from becoming a hindrance to the color of a lower layer when the colors of these layers are mixed. When an image formed on an OHP sheet is projected onto a screen, the chromaticity of the image projected on a screen through an OHP may differ from the expected if the transparency of toner is inferior, resulting in an undesired color. From this viewpoint, a high transparency is needed as well. In particular, humans are highly sensitive to a variation in hue angle of yellow and tend to easily recognize a variation in chromaticity of transmitted light. Therefore, the high transparency of a toner is particularly important.
Up to now, a monoazo compound has been known as a yellow pigment. The monoazo yellow pigment is excellent in color tint and coloring power of reflected light, so that it is desired to be used for a color toner. However, the monoazo yellow pigment has property of easily allowing the growth of primary particles and aggregation thereof upon the maturing of a pigment with heat after synthesis. For increasing the transparency of a toner, there is a need of reducing a dispersed particle size of a pigment in the toner. When the primary particles become large or aggregate, a problem will tend to occur with respect to the transparency of the toner. For solving the problem, several processes have been disclosed in the art, including a process having a so-called master batch step in which a pigment is mixed with a part of a binder resin in advance, a process in which a pigment is provided without being dried and used in a form of paste containing water to prevent the growth of particles therein, and a process in which a raw material containing a sulfonyl group and a raw material containing a benzimidazolone group are mixed in small amounts during a preparation of a pigment by a coupling reaction in order to keep the primary particle size of the pigment minimum (see JP 2001-166540 A, JP 08-234489 A, and JP 2000-63694 A).
However, even when the master batch is conducted or the pigment in a paste form is used, the dispersion of pigment particles is not yet sufficient, and thus an excellent color tint inherent in the monoazo yellow pigment is not sufficiently brought out when an image is projected to a screen through the OHP. The process for mixing the raw material containing the sulfonyl group and the raw material containing the benzimidazolone group in small amounts exerts an effect of preventing the growth in primary particle size of the pigment, but the pigment itself changes a color tint. Therefore, the excellent color tint inherent in the monoazo yellow pigment is not sufficiently brought out.
Further, there is another proposal in which the dispersion particle size of a monoazo yellow pigment in a toner is controlled (JP 2001-228653 A). In this case, however, the color tint of a transmitted image is not considered.
Furthermore, there is another proposal to improve the clarity of color hue and the transparency of an image on the OHP sheet by using of a pigment composition in which two or more different monoazo yellow pigments are mixed together for a toner (JP 10-171165 A). In this case, however, comprehensive studies are not sufficiently conducted, such as studies on the toner formulation and process for preparing the toner, so that there is still room for improvement.
Consequently, a yellow toner having a more excellent transparency and a more excellent color tint of an OHP transmitted image through an OHP sheet has been desired in the art.